media player 10 plays standard MPGs in widescreen

I downloaded Media Player 11, which didn't work properly and I rolled back to 10 but now my mpg videos are played back stretched to a wide screen. I can't find any way to re-configure playback. (the same videos play normally on my DVD player and played normally on Media player before I tried 11)

I had set up a system resore point before installing Media player 11 but it doesn't work, in fact none of my previous restore points work, it goes through the process, including restarting and then simply declares that it wasn't possible to restore; that is why I had to revert to 'rolling back', which is what happened when I tried uninstalling. None of the other suggestions work/apply.

thank you, everyone, for your help. I still don't understand what was wrong with my system restore points because there was one that I'd made myself which still didn't work. However the solution was what Hertz Van Rentyl suggested - i reinstalled WMP10 over the top of the one that wasn't working and it worked...